1. Field of the invention
The present invention relates to a semiconductor laser which can be used as a light source in a laser printer, a bar code reader and the like, and more specifically to a process for manufacturing a semiconductor laser generating a visible light having an oscillation wavelength of 680 nm or less.
2. Description of Related Art
Article No. 27p-R-12 in 1990 Autumn Meeting Transactions of the Japan Society of Applied Physics, page 927 discloses a semiconductor laser as shown in FIG. 1, and a method for manufacturing the same. This semiconductor laser includes a GaAs substrate 1 having an upper surface inclined at an angle of 5 degrees from a (1 0 0) plane toward a &lt;0 1 1&gt; direction (namely, (100) 5.degree. off to &lt;0 1 1&gt;). In a first crystal growth process of a MO-VPE (metal organic vapor phase epitaxy) process, a doubleheterojunction structure composed of a (Al.sub.0.1 Ga.sub.0.9).sub.0.5 In.sub.0.5 P active layer 4 (forming a light emission region) sandwiched between an n-type and p-type (Al.sub.y Ga.sub.1-y).sub.0.5 In.sub.0.5 P clad layers 3 and 5 having a forbidden band width larger than that of the active layer, is formed on the upper surface of the substrate, and furthermore, a p-type Ga.sub.0.5 In.sub.0.5 P layer 6 is grown on the p-type clad layer 5.
Thereafter, an oxide film (not shown) is deposited, and then patterned by a photoresist process so as to an oxide stripe which forms an etching mask for formation of a ridge stripe and a selective growth. And, the p-type clad layer 5 is etched to a middle in the thickness direction.
Then, a second crystal growth is performed so that a current block layer 10 is selectively grown using the remaining oxide stripe as a mask. Thereafter, the oxide film is removed, and a p-type GaAs contact layer 11 is grown on a whole surface. Finally, a pair of electrodes 12 and 13 are deposited on top and bottom surfaces of the substrate. Thus, the semiconductor laser is formed.
The conventional semiconductor laser as mentioned above has an oscillation light wavelength of 645 nm and an oscillation threshold of 80 mA to 90 mA. Another conventional semiconductor laser is disclosed in Japanese Patent Application Laid-open Publication No. Sho 62-200785.
In the above mentioned conventional process for manufacturing the semiconductor laser, the first crystal growth is terminated at the formation of the p-type Ga.sub.0.5 In.sub.0.5 P layer 6. Because of this, a doping characteristics of the p-type clad layer 5 becomes as a curve line "b" shown in FIG. 2. Namely, a saturated carrier concentration is as low as 3.times.10.sup.17 cm.sup.-3. Therefore, confinement of injected carriers within the active layer 4 in the doubleheterojunction structure becomes insufficient, so that an electron overflow becomes much, and therefore, the oscillation threshold becomes as high as 80 mA to 90 mA.